Stupid FET Question

[Lighty]

So how does this look? will these values work?

 

[BrownOut]

Looks like it will work. Best way to know is download ltspice and simulate it.

 

[Lighty]

Cool, will do.

Would the system be damaged if the load was not connected when powering up?

 

[BrownOut]

Cool, will do.

Would the system be damaged if the load was not connected when powering up?

It shouldn't.

 

[Lighty]

Ok, loaded it into LTSpice, and it doesn't work.....

The 2N7000 triggers and causes a voltage drop to the gate of the P-FET, 12V down to 2V, but the current on the load goes up a little but doesn't drop to zero, IE, the P-FET is not switching off, (open circuit).....

 

[BrownOut]

Ok, loaded it into LTSpice, and it doesn't work.....

The 2N7000 triggers and causes a voltage drop to the gate of the P-FET, 12V down to 2V, but the current on the load goes up a little but doesn't drop to zero, IE, the P-FET is not switching off, (open circuit).....

Post you .asm file.

 

[Lighty]

did you mean the .ASC file, from LTSpice?

 

[BrownOut]

Here is a corrected version. You're P-FET was upside down. The P-FET in this simulation isn't sourcing enough current to drive the 6-ohm load. I'll investigate later what parameters to change. Maybe someone with more experience can model a better transistor.

 

[Lighty]

Thanks for you help BrownOut, as well as everyone.

 

[Arkham00]

2n7000

Pay attention that the 2N7000 N-MOS is not a logic-gate MOS so it may not turn on properly because its Vgs min.
But because of the current in its drain will be very low, maybe the circuit can work in every case.

 

[Lighty]

The Vgs(th) is Min:0.8v Typ: 2.1V and Max: 3V (at 1ma).... Doesn't that make it within the criteria of a Logic Gate MOS?

 

[audioguru]

The Vgs(th) of a Mosfet is when it is barely turned on. It is almost turned off.

When a 2N7000 Mosfet has a Vgs of only 0.8V to 3V then its minimum current is only 1mA.
If its Vgs is 4.5V then it conducts a little more.
When its Vgs is 10V then it conducts as much as is shown in its datasheet which is not much because it is a tiny little Mosfet.

A "logic level" mosfet is turned on very well with a Vgs of 4.5V.

 

[colin mac]

Using an NPN transistor
**broken link removed**

 

[BrownOut]

Here is your circuit with a reasonable device chosen for the PMOS. The bigger device gives you the results you want. I don't really know anything about this device, I just picked it from a list by picking the lowest Rds(on) of the devices listed. You just need to find a device capable of driving your 6 ohm load. You only need to pour over alot of specs, and find the lowese Rds(on) and sufficient wattage. I'll leave that part up to you.

 

[Lighty]

Thanks for all the help, will try do some work on it this week.